Packaging machines for packaging pourable food products, such as fruit juice, wine, tomato sauce, pasteurized or long-storage (UHT) milk, etc., are known, in which the packages are formed from a continuous tube of packaging material defined by a longitudinally sealed web of packaging material.
The packaging material has a multilayer structure comprising a layer of paper material covered on both sides with layers of heat-seal plastic material, e.g. polyethylene, and, in the case of aseptic packages for long-storage products, such as UHT milk, also comprises a layer of barrier material defined, for example, by an aluminium film, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material eventually defining the inner face of the package contacting the food product.
The packaging material has a plurality of crease lines along which the packaging material is folded to obtain the packages. The final shape of the packages depends on the crease line pattern in the packaging material.
To produce such packages, the web of packaging material is unwound off a reel and fed through a sterilizing unit, in which it is sterilized, for example by immersion in a liquid sterilizing agent, normally a concentrated hydrogen peroxide and water solution.
Once the web has been sterilized, the sterilizing agent is removed, e.g. vaporized by heating, from the surfaces of the packaging material, and the web of packaging material so sterilized is maintained in a closed sterile environment, and is folded and sealed longitudinally to form the tube.
More specifically, the web of packaging material is fed vertically through a number of successive forming assemblies, which interact with the web to fold it gradually into a cylinder. More specifically, the forming assemblies comprise respective folding members defining a number of compulsory passages varying gradually in section from an open C to a substantially circular shape.
By interacting with the folding members, opposite lateral portions of the web are superimposed one on top of the other, so as to form the tube.
At a sealing station, downstream of the folding assemblies, the superimposed lateral portions of the web are heat sealed to each other to form a longitudinal seal of the tube.
The tube is filled continuously with the pourable food product and then sent to a forming and transverse sealing unit for forming the individual packages and in which the tube is gripped between pairs of jaws to seal the tube transversely and form pillow packs.
The pillow packs are separated by cutting the sealed portions between the pillow packs, and are then fed to a final folding station where they are folded mechanically into the final shape.
To ensure good transverse sealing of the tube of packaging material and correct folding of the pillow packs along the crease lines, the tube must be fed in a predetermined or desired angular position with respect to its own axis and to the structure of the packaging machine. When the tube of packaging material is sealed transversely, in particular by means of an ultrasonic sealing device, the superimposed lateral portions of the web must engage a respective groove formed in a counter element opposing an active element of the sealing device between which the packaging material is gripped under pressure. If not, this may result in an incorrect distribution of the contact pressures between the active element and the counter element of the sealing device and the packaging material, thus negatively affecting the quality of the seal.
In addition, if the tube of packaging material is twisted around its own axis with respect to the predetermined or desired angular position, it may happen that the crease lines are not aligned with the pairs of jaws of the forming and transverse sealing unit so impairing the forming of the packages. In particular, if the pairs of jaws fold the packaging material at regions thereof different from the creasing lines, the packages may have slightly curved longitudinal edges and, therefore, a bad visual appearance.
In known packaging machines, the angular position of the tube may vary, in actual use, from the predetermined or desired angular position, due to the lateral edges of the web not being perfectly straight, and due to the impact of the pairs of jaws on the tube.
To minimize the angular shift of the tube with respect to the predetermined or desired angular position, the folding member of one of the forming assemblies is connected to the structure of the packaging machine in angularly adjustable manner about the axis of the tube being formed, so as to enable adjustment of the angular position of the tube. This is done manually, however, by the operator at the start of the cycle and, if necessary, following routine checks of the packages coming off the machine.
The correction made by the operator therefore takes a relatively long time, normally in the region of a few minutes, which, given the high output rate of the packaging machines considered, amounts to a relatively large number of packages being rejected at the end of the cycle.